Yes, "bloat" which is frequently accompanied by "torsion" is a life-threatening condition which GR's (as well as many other big dogs) are prone to. There is no test that can be performed to tell which dogs might get this, and no scientifically determined cause. I've had two dogs with this, one made it, the other didn't.
The latest research seems to think that the ligaments which hold the stomach (which in the dog runs lengthwise of the body, as opposed to humans which is cross-wise) relax, allowing the stomach to turn. When this happens the contents cannot pass from either end, the blood pumped by the heart backs up and the dog usually dies of cardiac arrest. Why this happens is unknown. Some of the recommended things to do to prevent this include: feeding two small meals per day rather than one big meal, limiting exercise after eating, feeding food wet, etc. All are good things to do, but nothing is a guarantee.
I don't want to alarm you, but if you ever see your dog with a swollen stomach, trying to vomit unsuccessfully, restless and unable to settle down, ACT IMMEDIATELY!! You only have about two hours, so get the dog to the vet as fast as you can. This is NOT a condition to wait and see if it's better by morning--it won't be. Much better to be embarrassed and have to pay an after-hours fee unnecessarily than to risk losing your dog. Call your vet, explain what you see, and ask if it could be bloat. There is surgery than can be done which prevents this from recurring, since once a dog has bloated, it's likely to reoccur. They attach the stomach to the floating ribs which won't allow the stomach to turn. My first dog with bloat had this done and lived four more years to die a natural death at age 11. The second dog was being boarded--he was fine at 9 pm and dead in his run at 5 am. He was 9 and had no previous history--in fact, he was a picky eater all his life--the autopsy showed only a cup of food in his stomach. It does happen, so the best advice I can give you is to know the symptoms and act quickly if you observe something suspicious.
Bloat Links:
http://www.vet.purdue.edu/epi/bloat.htm